Magnetrons are widely used as powerful and compact sources for the generation of high power microwaves in a variety of applications. Such applications may include, but are not limited to, microwave ovens, telecommunications equipment, lighting applications, radar applications, and military and weapons applications, for example.
A typical conventional magnetron structure is a coaxial vacuum diode with a cathode having a solid cylindrical surface and an anode consisting of cavities forming an azimuthally periodical resonant system. In many designs, resonator cavities of various shapes are cut into the internal surface of the anode, for example, in a gear tooth pattern. During operation, a steady axial magnetic field fills the vacuum annular region between the cathode and anode, and a voltage is applied between them to provide conditions for microwave generation. Transverse electric-type (TE) eigenmodes of the resonant system are used as operating waves. Usually two types of oscillations are used, the π-mode (with opposite directions of electric field in neighbor cavities) and the 2π-mode (with identical directions of electric field in all cavities). The frequency of the generated microwaves is based in part on the number and shape of the resonator cavities, and the design features of the anode and cathode.
A cross-sectional view of a conventional well-known A6 magnetron modeled using the “MAGIC” particle-in-cell (PIC) code is illustrated in FIG. 1. As shown, a conventional magnetron comprises an anode 10, a cathode 20, which is a solid cylindrical structure, and resonator cavities 15. In this example, a waveguide 40 is located in one of resonator cavities 15 in order to extract the generated microwaves. A dielectric 40a also may be present in the waveguide 40. There are other ways known to those skilled in the art for extracting the microwaves as well, such as, for example, axially using diffraction output.
Electrons emitted from the cathode 20 form a solid flow drifting around a cathode with velocity determined by the applied voltage and magnetic field. When the azimuthal phase velocity of one of eigenmodes of the resonant system is close to the azimuthal drift velocity of the electrons, energy of electrons is transferred to this electromagnetic wave. As the wave gains energy, fields of the wave back-react on the electron charge cloud to produce spatial bunching of the electrons, which in turn reinforces the growth of the wave.
Magnetrons are either of the hot (thermionic) cathode type, which typically operate at voltages ranging from a few hundred volts to a few tens of kilovolts, or of the cold cathode type, with secondary electron emission or explosive emission, the latter of which are typically used in relativistic magnetrons, which operate at high voltage (hundreds kilovolts) and enable the generation of very high power microwaves.
For many applications, such as, for example, telecommunications, radars, but especially for military and weapons, it may be desirable to provide fast start of oscillations. The start time of oscillations of a magnetron is determined by two factors, 1) the start conditions, which give the initial impetus to the development of oscillations, and 2) the rate of buildup, that is, the growth rate of oscillations.
In a magnetron with a conventional solid cathode (with uniform electron emission), the initial noise level, which is about 10−10 of the energy of electrons, provides an initial impetus to the development of instabilities in the electron flow that is associated with the appearance of oscillations. This process may begin the forming of the electron flow modulation many tens of cyclotron periods later because of the relatively low noise level.
The rate of buildup is determined by an azimuthal electric field of the operating wave in the electron flow. In a magnetron with a solid cathode, that field is proportional to the thickness of the electron flow and equals zero on the metal cathode surface. Therefore, to provide a fast rise time of oscillations, increasing the thickness of the electron flow may be desirable. However, such an increase in thickness may lead to decreasing efficiency of the energy transfer. Moreover, attempts to increase the efficiency and output power of a conventional magnetron by increasing the voltage and magnetic field (that retains the closeness of phase velocity of the operating wave and drift velocity of electrons, which is the necessary condition for microwave generation, and decreases the thickness of the electron flow) ultimately may lead to degradation of output characteristics. This may occur because the azimuthal electric field of the operating wave, which is responsible for a capture of electrons to the anode, becomes too small.
It also may be difficult to generate long radiation pulse lengths with conventional relativistic magnetrons due to closure of the anode-cathode gap by plasma from explosive emission cathodes. Plasma interferes with the electromagnetic operation of the magnetron, either by creating a shorted current path, or by detuning the resonant cavities 15.
One approach that has been utilized in an effort to improve microwave production includes modifying the cathode surface to obtain a cathode with non-uniform emission that promotes a faster appearance of favorable modulation of the electron flow (“cathode priming”) than in the case of a cathode with uniform emission.
Another approach includes periodically perturbing the DC axial magnetic field by placing permanent magnets around the resonant system. This approach (“magnetic priming”) leads to increasing the electron flow modulation.
However, although these conventional approaches (cathode priming and magnetic priming) can provide a stronger initial impetus for the development of the electron flow modulation and thereby its faster development, they may not address many of the deficiencies and/or desirable features noted above. By way of example, and not limitation, the conventional approaches may not achieve sufficient shortening of the time to development of oscillations, which in part is determined by the rate of buildup. Moreover, these conventional approaches may not improve magnetron efficiency and/or address the issue of plasma closure.